Automotive brake systems have at least two channels for supplying brake pressure to the wheel brakes so that in the event of a failure in one channel, another channel will be operable to furnish braking to some of the wheels. The channels are configured in many ways such as one channel for front wheels and another for rear wheels, or one channel for each front wheel and another for rear wheels, or a diagonal split configuration. Ordinary brake systems supply these channels from pressure developed in a master cylinder which is usually augmented by a pressure booster arrangement. When the master cylinder is operated by the driver depressing the brake pedal the brakes are applied and remain applied until the pedal is released.
Antilock braking systems typically add to the ordinary brake function the ability to sense when wheel slip occurs and then release pressure to the slipping wheel and later reapply pressure when the wheel speed recovers. Also a pressure hold function is sometimes used in the antilock operation. To assure an adequate supply of brake pressure a separate pump may be used during antilock operation as a source of reapply pressure. A challenge in designing such systems is to provide a pump which is both economically feasible and adequate to provide the brake pressure demands of all the channels. An economical pump can supply a single channel but the simultaneous pressure application in more than one channel can not be assured. To assure a sufficient pressure supply an accumulator may be used to store pressurized brake fluid but this is an expensive solution to the problem. It is preferable to assure adequate pressure without the luxury of an accumulator.